LARGE VRANCEA INTERMEDIATE DEPTH EARTHQUAKES AND

Transcription

EARTH PHYSICS
LARGE VRANCEA INTERMEDIATE DEPTH EARTHQUAKES
AND SEISMIC MICROZONATION OF BUCHAREST URBAN AREA
LES GRAND SÉISMES DU VRANCEA DE PROFONDEUR INTERMÉDIAIRE
ET LE MICROZONATION D’AIRE URBAINE DE BUCAREST
NECULAI MÂNDRESCU, MIRCEA RADULIAN, GHEORGHE MÃRMUREANU,
BOGDAN GRECU
National Institute for Earth Physics, 12 Cãlugãreni Street, Ilfov,Romania
Email: nmandrescu@infp.ro; mircea@infp.ro; marmur@infp.ro; bgrecu@infp.ro
Received October 16, 2006
Le grand séisme qui a eu lieu dans la région de Vrancea, le 4 Mars 1977, a
produit l’écroulement de 32 haut bâtiments dans la parte centrale de Bucarest et une
autre dizaine de haut bâtiments ont été gravement avariés. On a généralement
supposé que la cause principale de cette destruction a été la proximité de la période
d’oscillation du bâtiment vis-à-vis de la période fondamentale de résonance
spécifique pour les conditions géologique au-dessous de la ville. Le but principal de
cette étude est l’analyse de l’influence des conditions locales sur la réponse de sol
dans Bucarest, pendant les grands tremblements de terre de Vrancea (M > 7) de
profondeur intermédiaire. Pour accomplir ce but nous avons utilisé deux sets de
dates: (1) dates géologiques, géotechniques et géophysiques, comprenant les
mesurages in situ des ondes transversales, et (2) les enregistrements d’accélération
des séismes du Vrancea de profondeur intermédiaire, produit entre 1977 et 2004. Les
résultats de notre étude concernant l’évaluation de la réponse locale, en utilisant les
contrastes d’impédance et facteurs d’amplification, mettent en évidence deux
caractéristiques majeurs de haut signifiance pour l’ingénieurs: (1) le discordance pour
le cas de la ville de Bucarest de la procédure standard qui limite la profondeur
d’investigation à 30 m pour établir les caractéristiques dynamiques du sol; la réponse
locale pendant les grands tremblements de terre de Vrancea est contrôlée par les
dépôts sédimentaires Quaternaires entières qui sont significativement plus grosses
que 30 m au-dessous Bucarest; (2) la difficulté de définir des zones avec des réponses
différentes. Ainsi, pour la zone urbaine de Bucarest et pour les tremblements de terre
de Vrancea de profondeur intermédiaire on peut parler plutôt des effets régionales
que ‘locales’.
1. VRANCEA INTERMEDIATE DEPTH EARTHQUAKES – THE MAIN SOURCE
OF SEISMIC HAZARD FOR BUCHAREST URBAN AREA
The Bucharest is among the most vulnerable European capitals to earthquake,
due to the seismic activity in the Vrancea region. That region, in which
earthquakes are located in a confined, isolated focal volume, beneath the Eastern
Rom. Journ. Phys., Vol. 52, Nos. 1– 2 , P. 171–188, Bucharest, 2007
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N. Mândrescu, M. Radulian, Gh. Mãrmureanu, B. Grecu
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Carpathians Arc bend, at intermediate depths (60–180 km), is characterized by a
persistent seismicity rate with an unusually high frequency of strong shocks (2–3
shocks with Mw > 7.0 per century) relative to such a small source active volume.
The focal mechanism for the largest Vrancea shocks are typically of reverse
faulting type with the T axis almost vertical and P axis almost horizontal (e.g.,
Radulian et al., 2000).
Among the former earthquakes those of 1471, 1620, 1681, 1738, 1802,
1829, 1838, 1893, 1894 stand out in terms of their effects. These events certainly
caused great damage, but information about them is scarce, mainly because there
was not a permanent observation network and also because of the relatively
reduced density of the population.
The first document referring to the effects of a Vrancea earthquake in
Bucharest dates from August 19, 1681 (Mw = 7.1), in the reign of ªerban
Cantacuzino (ªtefãnescu, 1901). According to the contemporary account “the
Earth shook so strongly as nobody had ever related”. The earthquake of June 11,
1738 (Mw = 7.7), destroyed the walls and tower of the Prince’s Court in
Bucharest. Many houses and churches were damaged and a “deep fracture” was
open near the town. A very strong earthquake occurred on October 26, 1802,
(Mw = 7.9), known by the contemporaries and lasting in the memory of the
subsequent generations as the “big earthquake”. The earthquake was felt over a
huge area, from Saint Petersburg to Greek islands and from Moscow to
Belgrade. During the earthquake all the church towers in Bucharest felt down,
and many churches and houses collapsed. The next Vrancea major earthquake
occurred on January 11, 1838, mentioned in several documents and in the
newspaper “Romania”, that started to be issued that time in Bucharest. A welldocumented description of the earthquake effects was reported by Gustav
Schüller (1882) immediately after the earthquake. Many of houses, especially of
stone, were destroyed (much less damage was reported for wooden houses) and
the royal palace was significantly damaged. Some documents indicated around
600 casualties and roughly the same number of injured people. Other significant
earthquakes in 19 century occurred on November 13 1864 and March 4 1894.
Last century two destroying earthquakes hit the city, in November 10, 1940
(Mw = 7.7) and March 4,1977 (Mw = 7.4). The more recent events of August
30, 1986; May 30 and 31, 1990 were recorded by a relatively large number of
instruments, providing this way important information on the Vrancea
earthquake characteristics.
The November 10, 1940 earthquake severely damaged many buildings and
the new 13-storey reinforced-concrete Carlton Hotel, sited in the central zone of
the city, collapsed, killing 267 people. Since other tall buildings with reinforcedconcrete frame suffered heavy damage, the authorities decided for the first time
in Romania to introduce rules for antiseismic building design. In a work issued
immediately after the earthquake, Beleş (1941) identified numerous buildings in
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Large Vrancea earthquakes and seismic microzonation of Bucharest
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the central part of the city, mainly of 6–8 storied, which needed urgent work of
consolidation. The ignorance of these recommendations and the superficiality of
the repairs had catastrophic consequences when the next major earthquake hit
the city on March 4, 1977. The author also mentioned: “for the same material
and execution, the buildings will behave worse as the tallness is higher” and, “as
concerns the dislocations of the foundation, we have almost nothing to notify in
Bucharest, and except a few isolated cases, the building basements were not
affected”.
The March 4, 1977 earthquake was the most destructive seismic shock that
hit the city in modern times. A number of 32 buildings of 8–12 storey collapsed
in the central part of city, while about 150 old buildings of 4–6 storey high were
strongly damaged. Almost all the collapsed buildings had been built between
1920 and 1940 without earthquake resistant design. These buildings suffered
greater or smaller damages during the November 10, 1940 earthquake and
during the second World war. Moreover, some of the buildings had been
submitted to important structural alteration, required by successive changes in
their vocation, while the initial design was ignored. The most of victims were
reported in Bucharest (1391 deaths and over 7576 injuries). The total value of
damage exceeded 2 billion US dollars, of which about 2/3 recorded in Bucharest.
The earthquake was recorded on the Romanian territory by a single station, in
Bucharest, by a SMAC-B type analog accelerometer and a Wilmot WS-1
seismoscope. The earthquake produced no permanent deformation, no slides or
collapses along the Colentina and Dâmboviţa river sides.
2. SEISMIC MICROZONATION OF THE BUCHAREST URBAN AREA
In earthquake prone areas the protection of buildings is mainly achieved by
certain paraseismic design norms and standards depending on the degree of seismic
hazard involved in the area under consideration, and on the type and function of
the building themselves. The seismic zoning maps have been elaborated, generally
on the basis of data collected on previous earthquakes and of geological and
seismotectonic research activities. These maps usually show the expected
seismic intensity; they cover the country’s entire territory and give the general
seismic characteristics, but they prove insufficient for a detailed planning.
In order to choose seismically safe zones for sitting most important or most
vulnerable component of urban and industrial development a detail investigation
and study of seismic microzoning is imperative. The seismic microzoning
methods currently employed developed by studying the impact of local
geological conditions upon the construction behavior during an earthquake. This
activity, promoted as a measure of antiseismic protection of constructions
stimulated the elaboration and improvement of design codes and standards for
constructions located in seismic regions.
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2.1. SHORT HISTORY
The seismic microzonation studies started in Romania more than 50 years
ago. Two important stages can be distinguished in the development of these
studies. The first one started in 1953 with the study by Ghica (1953) and was
followed by the works of Ciocârdel et al. (1964) and Mândrescu (1972). This
stage ends with the record of the first accelerogram of the strong Vrancea
earthquake of 4th March, 1977 which marks the beginning of the second stage
that continues up to the present day.
2.1.1. 1953–1977 interval
The seismic microzonation studies of the first stage were based on the
experience gained through evaluation of the effects of the crustal earthquakes
that took place in various seismic regions on traditional buildings, generally one
or two-floored and built of wood, brick, stone or adobe. Starting from the
assumption that foundation ground is very important for the behavior of the
buildings, the geological, geophysical, geotechnical and hydrogeological
particularities of some urban areas were thoroughly studied.
The first study on seismic microzonation in Bucharest was carried out by
Ghica (1953) at the Geological Department of the former Geological Committee.
The microzonation map (Fig. 1A) was drawn on the basis of the analogy
between the seismo-geological characteristics of the city of Bucharest and those
of other cities from other seismic areas and the theoretical evaluations related to
the response of the different types of rocks to seismic stress (Sieberg, 1937).
According to official regulation (STAS 2923/52), that time Bucharest belonged
to the area of VIIIth seismic degree. The author kept the seismic intensity
provided by the above mentioned standard for the meadows of Dâmboviţa and
Colentina rivers and reduced the intensity on the Băneasa-Pantelimon plain,
Cotroceni-Văcăreşti plain and Dâmboviţa-Colentina interstream by one degree.
He separated a transitional area (VII–VIII) in the center of the city on the reason
that the phreatic water between the loesslike deposits and the Colentina gravels
could cause settlements of the foundation ground, threatening the stability of the
buildings. A major contribution of the work is the recommendation to the
designer engineers to avoid location buildings with a period of oscillation
between 1.0–1.5s, as this is the fundamental period characteristic for the
Bucharest city area.
The study carried out at the “Project Bucharest” Institute (Ciocârdel et al.,
1964) was based on a huge amount of geological, geotechnical and hydrogeological data. At that time the standard in force (STAS 2923/63) placed Bucharest
in the VIIth degree of intensity area. The authors established, for each important
geomorphologic unity, a synthetic lithological column for the first 40 m from the
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Fig. 1 – The seismic microzonation maps of Bucharest urban area.
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surface. Among these, a “reference” lithological column was chosen, which was
representative for the geotechnical particularities of most of the city area. This
was associated to the VIIth degree of seismic intensity stated by the above
mentioned standard. The criterion for the separation of the microzones was the
seismic rigidity of the foundation ground and the influence of water table level,
according to Medvedev’s method (1960). The information regarding the seismic
waves velocity was taken from literature. As one can notice (Fig. 1B), the
authors preserved for most of the city the VIIth degree of seismic intensity and
increased by half, respectively one degree in certain areas from Dâmboviţa and
Colentina river meadows. On this map there are also marked the artificial fill
areas, with the recommendation to be avoided as building sites.
The Bucharest Geological Prospecting Company organized during 1970–
1972 geological research and geophysical measurement of the seismic wave
velocity in the Quaternary deposits, in order to establish the seismic rigidity of
the foundation soils. Records were made in more than 200 points all over the
city; there were collected and analyzed data regarding the lithological structure
and the geotechnical characteristics of the Quaternary deposits in over 2000
geological, geotechnical and hydrogeological boreholes (Popescu et al., 1964;
Mândrescu, Soare, 1970). At the time of the study there was also in force the
standard from 1963 (STAS 2923/63), according to which Bucharest belonged to
the VIIth degree seismic area.
The seismic microzoning map carried out by Mândrescu (Fig. 1C),
represented the key element in elaboration of the first seismic microzonation
standard (STAS, 8879/6-73) for Bucharest city area (Fig. 1D).
The analysis of the first three microzonation maps (Fig. 1A, B and C)
reveals two common features as follows:
– Seismic microzonation represents by definition the separation of an area
characterized by a certain degree of seismic intensity, into microzones of
different degrees. Each time, one starts from the reference level of intensity
established through official standards or norms for the respective city or area.
But, as showed before, the seismic zonation map can undergo changes in time,
as a result of either the knowledge level of the seismic phenomenon and its
impact on the built environment, or due to the seismic protection level which
society can afford at one time. For example, at the moment of the first seismic
microzonation study (1953), the city was considered to belong to the VIIIth
degree macrozone (STAS 2923/52) while at following the researches of 1964
and 1972 to the VIIth degree macrozone (STAS 2923/63). At present,
according to the Romanian standard of seismic zonation (SR 11100/1-93),
elaborated after the 1977 earthquake, the city belongs again to the VIIIth
degree macrozone;
– The separation into microzones leads to changes in the parameters for the
earthquake resistant design of the buildings, with effects on costs and of
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course on responsibilities. Taking into account the fact that the microzonation
map reflects the influence of the geological conditions at local scale (“local
response”), the boundaries of the microzones are, generally speaking, natural
ones. They are either changes in topographic aspect (the upper part of the
terraces, the base of the slopes, the limit of the plaines etc.) or differences
between the lithological structures of the deposits. But, obviously, the
geological conditions change abruptly very rarely. As a rule, the lithological
properties of the subsoil change in a continuous manner with distance. Under
these circumstances, the boundaries of the microzones will be settled after the
interpolation of the correction values or of the points where the parameters
have been recorded; the number of these observation points must be high
enough, so as their density on the surface unit should endow the separate
microzones with justification and credibility.
2.1.2. 1977–2006 interval
The evaluation of the 1977 earthquake effects in Bucharest proved that the
old tall buildings, with a reinforced concrete frame structure, situated in the
center of the city, were the most damaged. On the contrary, the rigid buildings of
the same height, the large-panel precast concrete structures and cast-in-place
reinforced concrete shear wall structures, as well as the brick houses with 1–2
floors, were the least affected. The location of the 32 blocks of flats that
collapsed during the earthquake are represented in Fig. 1D.
As one can see, most of the collapsed blocks were in the centre of the city,
both in the VIIth degree microzone and in the VII–VIIIth and VIIIth degree of
seismic intensity areas. Except for the blocks in Militari, Lizeanu and the
building of the Computed Centre of the Ministry of Transport and Telecommunication – which were new, all the other collapsed buildings had been built in
the 1920–1940 period, without any seismic protection precautions. After the
earthquake a team working for the National Council for Science and Technology
(CNST), proposed a new seismic microzonation map of the city (Fig. 2). The
separation of the microzones was made by converting the seismic intensity
(established as a result of the behavior analysis of a large number of buildings
during the earthquake) in acceleration, using, slightly modified, Shebalin’s
suggestions (Shebalin, 1975). The overestimated values of the acceleration, as
well as the concentric distribution of the microzones, do not correspond to the
geological particularities of the city. Moreover, the instrumental records of the
1986 and 1990 earthquakes in Bucharest refuted this delimitation of the
microzones. Taking into account all these, we think that this map shows
essentially the distribution of the effects of the earthquake on buildings rather
than the local site effects (Mândrescu and Radulian, 1999).
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Fig. 2 – The seismic microzonation map proposed by the NCST Report, 1977.
3. CRITERIA USED FOR SEISMIC MICROZONING MAPPING OF BUCHAREST
Two types of information have been used for new seismic microzoning of
Bucharest city: a) physical properties of the local setting and b) spectral analysis
of the strong-motion records of the Vrancea subcrustal earthquakes occurred
between 1977–2004.
a) The information about the regional and local natural conditions was
provided mainly by the geological, geotechnical and hydrogeological boreholes,
and by in situ geophysical measurements done on seismic profiles and in
boreholes, measuring the seismic wave velocity (Fig. 3). This data set allowed us
to elaborate some important maps showing geographical distribution of site
response. The predominant period (Fig. 4), seismic intensity correction (Fig. 5)
and amplification factor (Figs. 6 and 7) were computed taking into consideration
the acoustic impedance contrast of the city subsurface strata (Mândrescu et al.,
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Fig. 5 – Average seismic intensity correction (Medvedev’s method).
2006). The slight differences between the values of the above mentioned
parameters render evident the difficulty of separating microzones of different
seismic intensities or local amplifications.
b) Both in Bucharest and in the surrounding area, the bedrock does not
come to the Earth surface, so we have no direct information regarding the
characteristics of the seismic movement at bedrock level. The lack of some
“reference” records on the one hand, and the large extension of weakly
consolidated sedimentary deposits on the other hand, determined us to evaluate
the influence of the local conditions on the seismic movement by comparative
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analysis of all the records of each earthquake. First, we evaluate the average
response spectra of acceleration for each earthquake, taking into account all the
records available at all the stations in the city (Fig. 8). This spectrum is
considered to be characteristic for the average local conditions. It makes sense to
compute an average spectrum having in mind the similarity among the spectra
for every event at each station. Then, the individual acceleration response spectra
are represented relative to the average response spectrum for each earthquake.
The deviations from the average response spectrum are insignificant for 1986
event (Fig. 9A), 1990 event (Fig. 9B) and 2004 event (Fig. 10A).
The Vrancea moderate-size earthquakes (4.0 < Mw < 5.3), were recorded at
9 seismic stations (Bonjer, Rizescu, 1999). The number of recorded seismic
events differs from one station to other. Thus, 4 earthquakes were recorded at
CA, COS, CV and TIT stations, 6 at GB station, 7 at FOR station, 8 at INC and
FGG stations, and 11 at MG station. On each graph (Fig. 10B) the average
spectrum, calculated on the basis of the records of the 15 events is represented in
comparison with the average spectrum calculated on the basis of the earthquake
records at each station. The maximum amplification of the average spectrum of
the 15 events is found at 0.23 s or, if we take into account the shape of this
spectrum, the maximum amplification takes place between 0.23 s and 0.4 s.
Some differences, otherwise insignificant, are observed when comparing the
average spectrum computed on the basis of all the seismic events and the
average spectrum at each station, which can be explained by the variable number
of events per station, each with its own magnitude, azimuthally orientation,
depth and different spectral characteristics.
Two important issues come out from our analysis: (1) the spectral
similarity among different stations for a given earthquake, and (2) the local
deviations occur each time at different stations (at EREN for 1986 event, PND
for 1990 event, GB for the moderate-size earthquakes). The spectral analysis of
the records obtained at the stations in Bucharest reveals the difficulty of
separating microzones with different spectral amplifications.
4. THE NEW SEISMIC MICROZONING MAP OF BUCHAREST
The microzonation of any target area, including that of a city, requires
integration of that location into the seismic area established by an official
document in force. According to the seismic zoning map of the Romanian
territory (SR 11100/1–93) the city of Bucharest belongs to the 8th seismic degree
area (MSK-scale). This map (see inset from Fig. 11) shows, in terms of intensity,
the distribution of the seismic hazard all over the country. It relates to the
average ground conditions, defined as “a shallow superficial geological package,
with the velocity of S waves between 300–500 m/s”.
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For the seismic microzoning map of Bucharest we choose as support the
topographic map presented and described in a previous work (Mândrescu et al.,
2004). The map preserves certain elements from the original variant, such as the
elevation contour lines that point out the flat relief of Bucharest Plain, the
location of the now drained swamps and the former flow of the Bucureştioara
river. There are also shown the recent filling areas, sinkholes, and the erosion
witnesses. On this map there are represented a series of new elements, such as
the isolines that mark the fundamental period characteristic for the city area, and
the limits of the area in Dâmboviţa river meadow that is exposed to floods in
case the dam at the Morii Lake broke at a very large Vrancea subcrustal
earthquake (M > 7.0).
Considering the geographic distribution of the points where the corrections
of the seismic intensity and the dynamic amplification factors were computed, as
well as the locations of the stations where the considered earthquakes were
recorded, we come to the conclusion that on the city area there cannot be defined
microzones with seismic intensities different from the intensity established
through seismic zonation. Sandi and Borcia (2000) and Borcia (2006), come to a
similar conclusion after analyzing the acceleration spectra of the large Vrancea
earthquakes. We obviously mean by microzones those that can have a real practical
significance, requiring the change from a certain level of seismic protection,
which is the 8th degree, established through seismic zonation (STAS 11100/1-93),
to other levels, by dividing into halves (for 7th degree) or, by doubling the
acceleration (for 9th degree) to be taken into account for the earthquake resistant
design of the buildings, with the due responsibilities and costs.
5. DISCUSSION AND CONCLUSIONS
Before drawing the conclusions we would like to add some comments
related to microzoning in general and to the seismic microzonation of Bucharest
city in particular. We first to point out that microzoning should be seen as a
mean to take measures that will allow to avoid or at least to diminish the damage
and the number of casualties in earthquake prone areas. The microzonation map
is the result of a detailed research of the natural environment of an area for
which the level of the seismic hazard endanger has been established by the
seismic zoning map. A map showing the distribution of PGA (or other parameter
of the ground motion recorded during the earthquakes), cannot be considered as
a microzonation map, having in mind the very large variability of these
parameters. In order to accomplish this, the microzonation map must show the
distribution of the soil response to the action of the maximum possible seismic
event for that area. To develop into safety an urban area sited in earthquake
prone areas call above all for accommodate building activity to environmental
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conditions. Studies and seismic microzoning maps, elaborate taking into consideration the natural conditions, will of large interest for the present-day
evolution and especially for the future evolution of the urban area analyzed.
It is known that urban planning should not disregard the interaction
between two complex systems: the manmade system (the city itself) and the
natural system consisting of the geologic factors and processes that cause or
accompany a large earthquake. The causality connection between these two
systems call for a good co-operation between architects, designers, civil engineers
and local administration factors on the one hand, geologists, seismologists and
other geoscientists on the other hand. Unfortunately, the recommendations or
warnings given by geologists or seismologists are mentioned in technical
accounts or in scientific papers, but are not heeded by the ones they address to.
Serious consequences were in many cases brought about by ignorance or
incomprehension of geological information. Among the many examples of this
kind, we quote the Anchorage (USA), Yungay and Ranrahirca (Peru) zones and
Bucharest city (Romania).
The Anchorage zone was geologically investigated long before the 1964
Alaska earthquake. The undertaken investigations emphasized the presence of
clays locally called the “Bootlegger Cove clay”. The distribution area and its
characteristics were described in a geological report in which the activation
conditions of slides were also rated (Miller and Dobrovolny, 1959). Although
some administrative departments had this report and even used part of the data
included, the information were not taken into consideration and the hazard
implied by these deposits was ignored when the general fast developing
systematization of Anchorage was undertaken. The huge landslides and
settlements of the loose granular deposits triggered by the 1964 Alaska
earthquake (8.3 < M < 8.6) caused about 60% of the total losses.
Another example is supplied by the Yungay and Ranrahirca localities, in
zones threatened by avalanches. Geological investigations pointed out numerous
avalanches and landslides that had occurred not long before in the area. Yungay
itself was sited on such a stabilized avalanche. Had these conditions been known
and had adequate measures been taken before the 1970 earthquake (M = 7.5) the
calamity that struck the two localities and in which 18.000 casualties occurred
could have been prevented (Cluff, 1971).
In the first seismic microzonation study made in Romania in 1953, Ghica
recommended to take care when constructing tall buildings in Bucharest with
fundamental periods in the range of 1.0 s and 1.5 s. His advice was disregarded
by designers and civil engineers until the 4th March 1977 earthquake. Thus for
many high buildings built between 1953–1977 the values of basic design
parameters were four times lower than normal, if they had taken into account the
real natural conditions (See Table 1). The tall blocks of flats built in that period
are highly vulnerable and this could prove fatal in case of the next strong
Vrancea earthquake (M > 7.0).
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Table 1
Values of basic design parameters (k s, β, Tc, ψ )
Regulation
β
ks coefficient
VI
VII
I-42 and I-45
VII
IX
0,05
0,10
0,005
P. 13-63
0,025
P. 13-70
Tc
Ψ
0,3
1,00–1,50
1,0
0,60–3,0
1,0
0,03
0,05
0,08
0,60–2,0
0,4
1,00–2,00
P.100-78
0,07
0,12
0,20
0,32
0,75–2,0
1,5
0,15–0,35
P.100-81
0,07
0,12
0,20
0,32
0,75–2,0
1,5
0,15–0,35
1,00–2,5
1,5
1,0
0,7
0,15–0,65
P.100-91
and
P.100-92
0,8
0,12
0,20
0,32
ks = basic design coefficient represents the ratio of design ground acceleration (PGA or
EPA) to gravity acceleration, g;
β = dynamic amplification factor, defined for average ground conditions, depends on
natural period of structure T and on corner period, T c;
ψ = reduction coefficient which depends on structure ductility, redistribution capacity of
stresses, attenuation effects, etc.
Another example is given by the strong earthquake of 1986. This event was
recorded by 7 seismic stations in Bucharest. Since the record at the seismic
station EREN is different from the others, one assumed that this record is typical
for the northern Bucharest area, covered by “predominantly sandy soil profiles”
(Lungu et al., 2000). The other six sites are presumably characteristic for the
eastern, central and southern parts of the city, covered by “predominantly clayed
soil profiles”.
However, there are numerous and sound arguments (Liteanu, 1952;
Ciocârdel et al., 1964; Mândrescu, 1972; Radulian et al., 2000; Mândrescu et al.,
2004) to query the credibility of the accelerogram of 1986 event recorded at
EREN and the subsequent separation of the city area in two different regions.
We draw attention on the dramatic effects that can come out through considering
erroneous conclusions infered on false premises. This example, as well as the
preceding ones, argues for a continuous and constructive collaboration between
specialists from geosciences, design and civil engineering.
The seismic microzonation maps have a predictive character, but this refers
to the ground response to the future large seismic event, and not to the response
of the built environment. That is why one cannot speak of “validating”,
“confirming” or “matching” the seismic microzonation maps with/to those
showing the distribution of the damage done by an earthquake or another. We
draw attention on this aspect because after the 1977 earthquake, the finding that
“the most damage in Bucharest corresponded to the safest area” (Berg, 1977),
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was interpreted by some specialists in a wrong way, blaming the natural
conditions (local effects) for damages and disregarding the real cause, that is the
vulnerability of the built environment and the proximity of the building
fundamental period (T) and the characteristic period of the geological deposit
response beneath the city (Ts). It is known that none of the buildings that
crushed down in the centre of the city had been designed according to the
microzonation standard in force at that time (STAS 8879/6-73). We have to
reiterate some of our previous statements (Mândrescu, 1978, 1982; Mândrescu,
Radulian, 1999; Mândrescu, Zugrăvescu, 2000) – that, as long as in the cities
coexist old, traditional buildings, built with or without seismic precautionary
measures, and new buildings, made of new materials, with new technologies and
designed according to the provisions from official standards and regulations, we
cannot expect the seismic microzonation maps “match” the damage distribution
maps. Of course, the comparison of both types of maps could be relevant
provided the buildings whose response to the earthquake are analyzed were
located, designed and built in accordance with the requirements of the respective
microzonation map.
There are some things to be added in respect to the allegation that the
damage done by the 1977 earthquake was caused by “the focalization of the
seismic waves as a result of the deep geological structure” or by “the fault in the
Neogene layer” (NCST Report, 1977). Deep borehole information which we
used for the maps and geological sections in one of our previous works
(Mândrescu et al., 2004), pointed out the quasi-horizontal position of the strata
that form the sedimentary package of the Moesian Platform, which questions the
possibility of a focalization of the seismic waves. But, even in the case of a
hypothetical focalization of the seismic waves at 3–4 km depths, according to the
allegations in the above-mentioned report, it is difficult to explain the selective
character of the damage. As known, many blocks of flats, very close to those that
collapsed, met no serious damage.
Regarding the faults identified in the basement of the Moesian Platform
through boreholes and outside city seismic survey, they could be traced only in
pre-Neogene formations, and not in the Miocene so much the less in the Pliocene
ones. These are older faults, reactivated several times in connection with the
orogene movements that led to the building up of the Carpathians. The most
recent movements are considered to take place after Upper Meotian (about
3.5 Ma ago) when, according to some authors (Paraschiv, 1979) the gas and
petroleum accumulations were formed in the surroundings of the city of
Bucharest.
Regarding the fault slip, we would like to add that an “active” or “capable”
fault must have at least one of the following characteristics (CFR, Appendix A,
1980): a) the proof of a surface slip or of a slip close to the surface of the ground
at least once in the last 35,000 years or repeated slips in the last 500,000 years;
186
16
b) the existence of a micro seismic activity instrumentally determined precise
enough to prove a connection to the fault; c) the existence of a structural
connection with a capable fault, as defined at (a) and (b), such as the movement
of either of them could trigger the movement of the other as well. If we take into
account these characteristics, we will acknowledge that the fault considered by
some people to be responsible for the collapse of blocks OD-16 and Lizeanu and
of the Computer Centre of the MTTC does not meet the requirements of an
active fault. On the other hand, even if it were regarded as active, it is hard to
accept its simultaneous movement with the 1977 seismic event. As difficult as
that is, we must accept the idea that among the faults at the Moesian Platform
basement this was the only one to move at the 1977 earthquake and that the
effect of the slip was the fall of three blocks out of the several hundreds situated
on its hypothetical trace.
*
The seismic hazard for Bucharest urban area comes from the Vrancea
seismogenic region, where 2–3 large earthquakes (M > 7.0) occur per century.
According to the seismic zoning map of Romanian territory (SR 11100/1-93)
Bucharest city belongs to the 8 th macrozone degree (MSK-scale).
The main result of the seismic microzonation study for Bucharest city
emphasizes seemingly in a paradoxically way the impossibility of delimiting
certain microzones with different “response” to the strong subcrustal Vrancea
earthquakes. The slight differences between the values of the computed
parameters are confirmed by the similarities in the acceleration response spectra
of each earthquake recorded by the different seismic stations in Bucharest. The
good correlation between the result obtained by using these two data sets are
mainly due to the quasi-uniform geological peculiarities over the entire city area,
with almost horizontal strata and insignificant lateral inhomogeneity; there is no
strong enough lithological differences (acoustic impedance contrast) to be
reflected in different site response.
Based on our analysis we conclude that the seismic source, travel path and
site effects will result in similar response over the entire city area in case of
strong Vrancea intermediate-depth earthquakes; thus, for Bucharest urban area
and strong subcrustal Vrancea earthquakes (M > 7.0), one can refer rather to
“regional” than “local” effects.
The differences are determined by the increasing thickness of the resonant
layer from south to north and by the increase in the fundamental period
characteristic in the same direction, from 1.0 s in the southern part of city, to
about 2.0 s in north (Figs. 4 and 11).
The power spectrum densities of 1977, 1986 and 1990 earthquakes, point
out a maximum dynamic amplification around 2.2 s period which could become
a serious threat for the tall buildings in Bucharest, in case of a large Vrancea
intermediate-depth earthquake (M > 7.0).
17
187
Finally, we want to draw attention on two crucial conclusions as concerns
the microzonation analysis for Bucharest city: 1. It is completely inadequate to
apply the usual procedure which limits the investigation of dynamic
characteristics of soil at 30 m depth in order to establish the local response
(Borcherdt, 1994; EUROCODE 8 CEN,1994; Wirth et al., 2003); 2. The
unstationary of the dynamic amplification process shows that the information
provided by the study of the weak and moderate-size earthquakes cannot be
extrapolated to anticipate the local response in case of strong subcrustal Vrancea
earthquakes.
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Fig. 8 – Average acceleration response spectra (A) and average power spectra density (B), for the
analyzed earthquakes.

LARGE VRANCEA INTERMEDIATE DEPTH EARTHQUAKES AND

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